1. Field of the Invention
The present invention relates to the drive train of the four wheels drive vehicle, and more particularly, to a diagnostic system for rotational speed sensors incorporated in the drive train of the four wheels drive vehicle having a central differential device.
2. Description of the Prior Art
In the drive train of the four wheels drive vehicle including a transmission such as a combination of a fluid torque converter and a speed change gear mechanism and a central differential device for distributing rotational power supplied from an engine through said transmission between a front wheel drive shaft for driving a pair of front wheels and a rear wheel drive shaft for driving a pair of rear wheels, the central differential device generally includes a lock-up clutch adapted to be selectively engaged to lock up the power differentiating operation thereof between the front wheel drive shaft and the rear wheel drive shaft so that the central differential device operates as a unitary body firmly connecting the front wheel drive shaft and the rear wheel drive shaft with the preceding speed change gear mechanism in order to meet with such a trouble that the traction of the vehicle is totally lost by either the front wheel or the rear wheel slips as caught on an icy road surface or in mud. The selective engagement of such a lock-up clutch in the central differential device can be automatically controlled so that it is automatically engaged when one of the wheels begins to slip if the rotational speed of at least two of an input rotational member and two output rotational members of the central differential device is individually detected by a rotational speed sensor and the output signals from these two rotational speed sensors are compared with one another by an appropriate comparison means.
The rotational speed sensors for such a purpose are available in various constructions such as a type in which variations of the permeability of gear teeth provided on a rotational member are picked up by an electromagnetic sensor, a type in which a reed switch opens and closes to generate a pulse signal in accordance with rotation of a magnet mounted on a rotational member, etc.. These rotational speed sensors are of course not perfectly immune to failure. If such a rotational speed sensor fails particularly in the modern four wheels drive vehicle equipped with a computer controlled automatic transmission, it may happen that the high performance to be available by such a computer controlled automatic transmission is seriously damaged without being noticed by a driver who is not very skilled in driving the vehicle.
In the art of automobile it was conventionally known to provide two sets of rotational sensors for a single rotational system to detect its rotational speed under comparison of two output signals from those two sensors so that if one of the two rotational speed sensors fails the failure is immediately detected. Such an art is not usable for the two rotational speed sensors adapted to detect the rotational speed of two of the three rotational members including an input rotational member and two output rotational members of the central differential device in the drive train of the four wheels drive vehicle, because the relation between the rotational speed of those two rotational speed sensors occasionally varies so much if one of the four wheels slips on a slippery road surface or in mud that a failure detecting means for the rotational speed sensors would be unduly triggered to dispatch an alarm when the rotational speed sensors have not yet failed.